Environmental and geopolitical concerns have motivated improvement on vehicle designs both to achieve better fuel efficiency and to reduce environmental impact. Recent years have seen development of hybrid vehicles in both automotive and railway industries. Hybrid vehicles operate on two or more energy sources, typically combining an energy storage system (e.g., batteries) or fuel cells with an on-board combustion engine.
In the automotive industry, Toyota® Prius™ brand hybrid cars have been the most successful. A Toyota® Prius™ can rely solely on a nickel-metal hydride (NiMH) battery to drive an electric motor in low-power conditions and switch to a gasoline engine in high-power conditions or when battery charge is low. In addition, the NiMH battery can be recharged by both the gasoline engine and a regenerative braking system. These and other features allow the Toyota® Prius™ hybrid vehicles to achieve relatively low levels of fuel consumption and carbon emission. All-electric (or plug-in type) automotives are also in the works but very few are commercially available.
The railway industry has also taken a definitive step towards more environmentally friendly locomotive designs. While all-electric railcars have been operated for decades, typically as subway or light-rail passenger carriers, they almost invariably have to rely on either an electric “third rail” or overhead cables for a continuous supply of power. However, the vast majority of railroads do not have third rails or overhead cables to accommodate electric locomotives, and costs of converting existing railroads are prohibitively high. In the United States, it was estimated that it would cost as much to electrify a railroad as it cost to build it in the first place. Overhead lines and third rails require greater clearances, and the right-of-way must be better separated to protect the public from electrocution. Therefore, recent innovations have been focusing on hybrid locomotives. Although there has not been a consistent definition of the term “hybrid,” all existing hybrid locomotives appear to build upon the traditional diesel-electric locomotive platform and include one or more energy storage mechanisms to receive excess energy from the diesel engine or regenerative braking or both.
Among the most notable hybrid locomotives are those developed by East Japan Railway Company (or “JR East”), General Electric (GE), and Railpower Technologies (or “Railpower,” a Canadian company). JR East's hybrid locomotive (2003 test design) included two 65-kilowatt fuel cells and six hydrogen tanks under the floor, with a lithium-ion battery on the roof. The JR East test train was capable of 60 mph with a range of 30-60 miles between refills. The GE hybrid locomotive is essentially a diesel-electric locomotive redesigned to capture the energy dissipated during braking and store it in a series of lead-free batteries. The stored energy can be later used on demand to reduce fuel consumption (reportedly by up to 15%). Railpower's hybrid locomotives include switchers known as “Green Goats” and “Genset” type locomotives. The Green Goat hybrid switchers are each powered by a small generator and a large bank of batteries. The Genset locomotives are powered by between two and four smaller diesel engines each, and they achieve part of their fuel savings and efficiency by turning engines on only as needed and shutting them down in low-power or idle conditions.
FIG. 1 shows a traditional design of a diesel-electric locomotive 100 based on which the various hybrid locomotive designs have been proposed. The traditional diesel-electric locomotive 100 typically comprises a diesel engine 102 that burns diesel fuel to power a generator 104. A high-voltage cabinet 106 regulates the electric current produced by the electrical generator (or alternator) 104 to drive a number of traction motors 108. The diesel engine 102 is referred to as the prime mover, while the electrical generator 104, the traction motors 108 and any interconnecting apparatus are collectively considered a power transmission system. Compared to this traditional diesel-electric locomotive 100, GE's hybrid design merely adds regenerative braking (with a limited energy recovery and storage capacity) to improve fuel efficiency of the diesel engine 102. Railpower's Genset design essentially splits the one diesel-electric engine 102 into two or more smaller engines and switches each engine on demand. JR East's hydrogen hybrid locomotive operates on essentially the same principle as the diesel-electric locomotive 100 but burns hydrogen fuel instead of diesel fuel.
Since the above-mentioned hybrid locomotives are still in development or early commercial deployment, their environmental contribution or commercial success is yet to be fully appreciated. These existing hybrid locomotive designs, however, do share one common feature—they still require on-board internal combustion engines as a direct or indirect power source and therefore still rely on fuels such as diesel or hydrogen. As a result, the existing hybrid locomotives either cannot truly eliminate carbon emissions produced by burning fossil fuel or have to rely on cleaner fuels at great expense. To date, there has not been any serious attempt at developing a battery-powered, all-electric locomotive with sufficient energy capacity and horsepower for commercial rail transport operations.
Generally, all-electric locomotives require configuring long strings of batteries in parallel to meet the energy storage requirements of trains. However, when batteries are configured in long strings, their cells do not typically perform uniformly. For example, when lead-acid batteries are configured in long strings, their charge levels typically become non-uniform. Generally, these batteries are balanced by overcharging them. However, overcharging lead-acid batteries typically results in problems such as gassing, drying out, and early battery failure. Accordingly, there is a need for batteries that perform uniformly when configured in long string configurations.
Even with improved batteries having high level of charge acceptances configured in long parallel strings, the inherent physical and chemical characteristics of these batteries nonetheless limit the energy storage system's ability to recapture a high horse power locomotive's via braking power. For example, a six-axle locomotive may generate 4,000 horsepower from regenerative braking, however, even batteries with high charge acceptance rates typically cannot accept more than 1,000-2,000 horsepower even in multiple parallel strings for long braking intervals. As a result, excess regenerative energy is lost. Simply increasing the amount of batteries used in an energy storage system to recapture the excess power is not a viable solution, because of the large amount of weight additional strings of batteries add to the train's load. Specifically, the additional weight of long strings of batteries may result in a net inefficiency. Further, federal regulations limit the total amount of weight allowed on an axel of a railcar. Accordingly, there is a need to manage a locomotive's regenerative power generation over time (energy) in order to accommodate the physical and chemical characteristics of an energy storage system.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current locomotive designs.